Coating articles of magnesium or magnesium base alloys



COATING ARTICLES F MAGNESIUM OR MAGNESIUM BASE ALLOYS Jack Ellwood Cuthbert Currah, Reddendale, Ontario, and Alfred Rudin and Max Morf, Beloeil, Quebec, Canada, assignors to Canadian Industries Limited, Montreal, Quebec, Canada, a corporation of Canada No Drawing. Application October 22, 1956 Serial No. 617,287

Claims priority, application Canada March 17, 1956 8 Claims. (Cl. 20456)' This invention relates to a method of producing coatings of a protective nature on the surface of articles of magnesium or magnesium base alloys. ticularly relates to an improved method of electrolytically coating the surfaces of articles of magnesium or magnesium base alloys and to the electrolytic bath used in said method.

Magnesium or magnesium base alloys exhibit good resistance to atmospheric corrosion, but are attacked relatively rapidly by most neutral and acidic salt solutions. The corrosion appears to be of the hydrogen-evolution type, i.e., hydrogen is liberated at cathodic areas and magnesium goes into solution from anodic areas. extent of corrosion depends on the corroding medium, the composition of the alloy and the time of exposure. Magnesium and magnesium base alloys are normally strongly anodic and have a pronounced tendency to dissolve when in electrical contact with other metals in an electrolyte. All commonly used metals will cause gal vanic corrosion of magnesium or magnesium base alloys in a conductive solution.

Most paints and varnishes do not'adhere well to untreated magnesium or magnesium base alloys because magnesium hydroxide, which is formed as a corrosion product, is strongly alkaline and causes peeling of the It more par- The paint film. Magnesium and magnesium base alloys are usually protected by painting after a chemical or electrochemical surface treatment.

Several good anodic treatments have been developed recently but none of these treatments has produced a corrosion-resistant coating which does not spall, crack or flake on bending.

It is an object of this invention to provide a method of producing a protective coating upon the surface of articles of magnesium or magnesium base alloys, which coating is an improvement over coatings heretofore available.

A particular object is to provide a method of producing a protective coating upon the surface of articles of magnesium or magnesium base alloys, which coating is uniform in thickness and appearance and resistant to corrosion, and which does not spall 0n bending or flexing of the article.

A further object is to provide an electrolytic bath for use in such a method.

Other objects of this invention and advantages thereof will appear hereinafter.

Broadly speaking, the aforesaid objects are accomplished by treating electrolytically articles of magnesium or magnesium base alloys in an aqueous solution comprising as essential ingredients an alkali metal borate and a member of the group consisting of hydrogen peroxide, soluble peroxygen compounds and compounds which yield hydrogen peroxide in solution.

It has been found that a protective thin layer of black or grey appearance can be obtained on articles of mag- 2,926,125 Patented Feb. 23, 1960 nesium or magnesium base alloys by treating them electrolytically in an aqueous solution comprising as essential ingredients an alkali metal borate and hydrogen peroxide or a compound which yields hydrogen peroxide in solution. The so-produced coating securely adheres to the surfaces to which it is applied, and is very resistant to corrosion.

In treating articles according to the invention, either direct current or alternating current may be used. The coatings produced by the treatment with direct current form more rapidly, efiect a more even coverage and exhibit a better appearance than those produced with the aid of alternating current. When alternating current is used, the article is, of course, anodic during a portion of the cycle. The cathode may consist of any suitable material such as magnesium, aluminum, stainless steel, copper, graphite, etc.

Before carrying out the coating process, the articles of magnesium or magnesium base alloys to be coated, are suitably cleaned in conventional manner according to the nature of the foreign matter to be removed. Thus, the removal of oil and grease, and of some of the usual chemical finishes, may be effected by immersing the articles in a hot alkaline stripping solution comprising, for example, caustic soda and diethylene glycol.

It is desirable to activate the surface of the article to be coated with acid as a preliminary step before coatsium base alloys which have not received any special treatment may be satisfactorily prepared for the electrolytic treatment of the invention, by conventional mechanical methods.

The electrolytic coating solution of the invention may be fortified with certain other ingredients thereby producing still better coatings. For example, the addition of ingredients such as alkali metal oxalates or dextrin reduces the tendency of the article to become pitted during the anodic treatment.

Borax, sodium metaborate or a mixture of boric acid and sodium hydroxide may be used alone, or in combination, as the source of borate ions. Good coatings are always formed when any one or a combination of these chemicals is present in solution in a concentration equivalent to 0.5 g./l. B 0 or greater. It is preferable to operate with a borate concentration considerably higher than the minimum values in order to obtain a good coating within a shorter application time.

At least a detectable trace of peroxide is required in the electrolyte for coating formation. When larger amounts of peroxide are used, there is a tendency for the anode to become pitted and for the coating to develop a black crust or not to be adherent. This crusting and pitting may be avoided by working at lower curirent density. While relatively concentrated peroxide solutions may be used it is preferable to use moderate concentrations for economical purposes since higher permissible current densities result in a shorter coating time. The maximum values of the current densities or voltages used are affected by solution composition and degree of agitation of the electrolyte.

The lowest pH at which a satisfactory coating can be formed is about 8.0, but the quality of the coating appears to improve with a higher pH. The upper pH limit is the pH at Which the solution precipitates.

Coatings can be formed at a temperature range extending from the freezing point of the solution to about 85 C., depending on the kind of articles to be coated. It is preferable, however, that the coating process takes place at a temperature range extending from 20 C. to 30 C.

The solution used for carrying out the coating treatment according to the invention may be prepared in various ways depending upon the choice of ingredients, and the pH may be adjusted to the desired limit by the addition to the solution, if necessary, of an alkali such as sodium hydroxide.

Conventional magnesium racks and clamps may be employed for carrying out the electrolytic process. It is essential that the racks or clamps make good contact with the articles to be coated, since the coating acts as an insulator. A brief dip in dilute nitric acid or fluoridecontaining solutions inhibits them from being coated and therefore keeps them clean. This feature is an advantage since no current need be lost in coating them.

The following table sets forth examples of preferred operating baths for the production of coatings according to the invention.

alloy article, which comprises immersing the article as the anode into a solution having a pH of at least 8 and consisting essentially of water, an alkali metal borate in a concentration of at least 0.05% by weight calculated as diboron trioxide, and a substance selected from the group consisting of hydrogen peroxide, soluble peroxygen compounds and compounds which yield hydrogen peroxide in a concentration of at least 0.001% by weight calculated as hydrogen peroxide, while passing an electric current through the solution.

2. A method of producing a corrosion-resistant coating on the surface of a. magnesium or a magnesium base alloy article, which comprises immersing the article as the anode into a solution having a pH of at least 8 and consisting essentially of water, an alkali metal borate in a concentration of at least 0.05% by weight calculated as diboron trioxide, a compound which yields hydrogen peroxide in solution in a concentration of at least 0.001% by weight calculated as hydrogen peroxide, and material selected from the group consisting of dextrin, alkali metal oxalates, and mixtures thereof, while passing an electric current through the solution.

3. An electrolytic bath solution having a pH of at Table l Electrolyte Anodic current deusity 10- 0 0 a i. Q E o 8 3 2 =1 Panel 3 it 2 E f: a g g r; 5 Remarks Nos. 0: 6,, 9. B 53*:1 'o 3 5 3 3 "1M mt. 11.. Om cs @A a. 00 a has a; a u h o 3 4: o Bag 3: era: a": me g; E! 5 g 5 5' 3 5 539, 1: :gg i c! '5 o "2 H we 52 a? E? 53 5 age s a a s5 Eb Q 9 en an 01-4 El) 0 G E '3 :2 m in 5: n-t Q hi E; 5 i=1 5 2. 5 1 7. 5 10. 5 0. 4 0. 6 28 5- 8 +1 g./l. M83003. 2. 5 l 7. 5 1.2 0.6 7 8 +1 g.l1. Nmoot. 6 16 2. 5 0. 8 17 7-10 2 18 8. 0 2. 0 1. 6 21 10 6 9. 8 1. 1 0. 4 5 10 20 9. 1. 3 0. 4 19 10 2 20 9. 5 2. 4 0. 4 11 10 +g.ll. Oxalic Acid. 4 0. 02 10. 2 2. 5 0. 3 7 10 2O 12. 1 2. 6 2. 5 5 6 33 4 20 9. 8 1. 5 1. 4 0. 9 4 0.75 10 0.6 0.6 4 7-15 34 4 0. 8 10. 6 2. 3 0. 5 10 5-22 2. 5 1 7. 5 1.3 0.7 7 8 g./l. N11 00:

(sodium carbonate). 14 14 2 0. 8 9. 8 l. 3 0. 4 2 12 Amp./cm.=amperes per square centimeter. Mins.=Minu es.

All panels in Table I were made of an alloy containing approximately 3% aluminum, 0.45% manganese, 1% zinc and 95.55% magnesium. Other magnesium base alloys of widely differing compositions and with different alloying elements have been coated satisfactorily by the described procedure.

The corrosion-resistant finish described above is dried by simply allowing it to stand in the air, or by air blasting or gentle heating and thereafter can be further improved by the application thereon of a suitable top-coat which will increase its abrasion resistance and its resistance to acids and salts.

Among the finishes that can be applied, the more suitable are selected from the group consisting of paints, varnishes and resins, and, more specifically, zinc chromate primers and commercial spar varnishes.

It is to be understood that the invention is not limited to the specific details before described but may be carried out in other ways without departure from its spirit.

What we claim is:

l. A method of producing a corrosion-resistant coating on the surface of a magnesium or a magnesium base G.ll.=grams per litre. Ml.ll.=millilitres per litre.

least 8 and consisting essentially of water, an alkali metal borate in a concentration of at least 0.05% by weight calculated as diboron trioxide, and a substance selected from the group consisting of hydrogen peroxide, soluble peroxygen compounds and compounds which yield hydrogen peroxide in solution in a concentration of at least 0.001% by weight calculated as hydrogen peroxide.

4. An electrolytic bath solution having a'pH of at least 8 and consisting essentially of water, an alkali metal borate in a concentration of at least 0.05 by weight calculated as diboron trioxide, a compound which yields hydrogen peroxide in solution in a concentration of at least 0.001% by weight calculated as hydrogen peroxide, and material selected from the group consisting of dextrin, alkali metal oxalates, and mixtures thereof.

5. A method as claimed in claim 1 wherein a temperature range extending from the freezing point of the solution to about C. is maintained during the entire coating time.

6. A method as claimed in claim 1 wherein a temperature range extending from 20 C. to 30 C. is maintained during the entire coating time.

I! o 6 7. A method as claimed in claim 2 wherein a tem- References Cited in the file of this patent perature range extending from the freezing point of the UNITED STATES PATENTS solution to about 85 C. 1s mamtalned during the en- 2,206,028 Buzzard July 2 1940 are coatlng time.

8. A method as claimed in claim 2 wherein a tem 5 2276353 Thompson 1942 perature range extending from 20 C. to 30 C. is main- FOREIGN PATENTS mined during the entire coating time. 815,155 France July 7, 1937 

1. A METHOD OF PRODUCING A CORROSION-RESISTANT COATING ON THE SURFACE OF A MAGNESIUM OR A MAGNESIUM BASE ALLOY ARTICLE, WHICH COMPRISES IMMERSING THE ARTICLE AS THE ANODE INTO A SOLUTION HAVING A PH OF AT LEAST 8 AND CONSISTING ESSENTIALLY OF WATER, AN ALKALI METAL BORATE IN A CONCENTRATION OF AT LEAST 0.05% BY WEIGHT CALCULATED AS DIBORON TRIOXIDE, AND A SUBSTANCES SELECTED FROM THE GROUP CONSISTING OF HYDROGEN PERODIXE, SOLUBLE PEROXYGEN COMPOUNDS AND COMPOUNDS WHICH YIELD HYDROGEN PEROXIDE IN A CONCENTRATION OF ALT LEAST 0.001% BY WEIGHT CALCULATED AS HYDROGEN PEROXIDE, WHILE PASSING AN ELECTRIC CURRENT THROUGH THE SOLUTION 